Aquifers that are suitable for CO2 disposal typically have caprock layers made up of low permeability sedimentary rocks like shale, anhydrite, or salt. Calcite and siderite-rich caprock layers are generally not ideal, because calcite and siderite are carbonate minerals that can react with CO2. The reaction of CO2 with these minerals can dissolve and degrade the caprock, compromising its sealing properties over time.